TITLE: GCN CIRCULAR NUMBER: 40810 SUBJECT: Konus-Wind detection of GRB 250620C DATE: 25/06/22 10:03:09 GMT FROM: Dmitry Svinkin at Ioffe Institute dmitrysvinkin@gmail.com

D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia, A. Tsvetkova, M. Ulanov, and T. Cline on behalf of the Konus-Wind team, report:

The bright,long-duration GRB 250620C (Fermi-GBM detection: The Fermi GBM team, GCN 40794; Mukherjee, GCN 40799; SVOM-GRM detection: Wang et al., GCN 40807; IPN triangulation: Svinkin et al., GCN 40809) triggered Konus-Wind at T0=58026.172 s UT (16:07:06.172).

The burst light curve shows a multipeaked structure which starts at ~T0-0.2 s and has a total duration of ~39 s. The emission is seen up to ~10 MeV.

The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB250620_T58026/

As observed by Konus-Wind, the burst had a fluence of 7.87(-0.47,+0.43)x10^-5 erg/cm2, and a 64-ms peak flux, measured from T0+1.344 s, of 4.05(-0.28,+0.28)x10^-5 erg/cm2/s (both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst (measured from T0 to T0+39.424 s) is best fit in the 20 keV - 15 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -1.00(-0.03,+0.03), the high energy photon index beta = -2.83(-0.42,+0.21), the peak energy Ep = 363(-17,+19) keV (chi2 = 116/97 dof).

The spectrum near the maximum count rate (measured from T0+0.256 to T0+1.536 s) is best fit in the 20 keV - 15 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.71(-0.05,+0.05), the high energy photon index beta = -2.44(-0.14,+0.12), the peak energy Ep = 618(-45,+47) keV (chi2 = 85/59 dof).

All the quoted errors are at the 68% confidence level. All the quoted values are preliminary.

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